In charged-particle-beam (CPB) projection apparatus based on a partitioned pattern-transfer method, the pattern field of a reticle or a mask (collectively referred to as a "mask") is divided (partitioned) into multiple mask subfields. The pattern on the mask is "transferred" (i.e., projection-exposed) onto a sensitive substrate, such as a wafer, sequentially one mask subfield at a time. The respective projected images of the mask subfields ("transfer subfields") on the substrate are "stitched" together to eliminate the partitions and produce an entire pattern, faithful to the mask pattern, on the substrate.
During conventional transfer processes, the image in each transfer subfield on the substrate is often blurred due, e.g., to lens aberrations, and/or to deflection aberrations caused when the charged-particle beam is deflected.
FIG. 4(a) illustrates a transfer subfield WSf formed by a conventional method on a substrate surface. Portions of the image of the corresponding mask subfield within the transfer subfield WSf are blurred. To illustrate the blurring, the transfer subfield WSf of FIG. 4(a) comprises twenty-five (25) rectangular pattern segments arranged in five vertical columns and five horizontal rows. The relative degree of blur at each of the pattern segments is represented by the symbols b0 through b5, with b0 representing the least amount of blur and b5 representing maximal blur. In FIG. 4(a), the contour of each pattern segment is depicted by six different types of lines representing relative degrees of blur. The wider the line outlining a particular pattern segment, the greater the blur.
Whenever the particle beam of the CPB projection apparatus is focused on the center of a transfer subfield WSf, the blur of the corresponding pattern segment at the center of the transfer subfield is usually minimal (b0), as shown in FIG. 4(a). Due to any of various aberrations such as distortion, coma, and astigmatism, the blur with which pattern segments are projected tends to increase with increasing distance of the subject particular pattern segment from the center of the transfer subfield WSf. Spherical aberration and chromatic aberration can also occur on the optical axis of the charged-particle beam, which can result in blurring of pattern segments in the center of the transfer subfield.
According to one conventional way in which to reduce blur, the focal point of each transfer subfield is axially displaced from the substrate surface so as to reduce axial spherical aberration and field curvature. Such a remedy tends to minimize blur of pattern segments that are located in regions of the transfer subfield between the center and the periphery of the transfer subfield WSf, as indicated in FIG. 4(b). As a result of such a focal-point adjustment process, the degree of blur in each projected transfer subfield has a more limited range, typically from b1 to b3. Thus, the extreme of blur represented by the symbol b5 of FIG. 4(a) is not found in the transfer subfield shown in FIG. 4(b).
The pattern segments contained in each individual mask subfield of an actual mask pattern usually differ from one another. As a result, the relative position, within each mask subfield, of minimum-linewidth features (which must be exposed with the highest accuracy) can vary from one mask subfield to another. If the focal-point adjustment scheme shown in FIG. 4(b) is uniformly applied to all the mask subfields, problems can arise in which certain minimum-linewidth features, when transferred to the respective transfer subfield on the substrate, have insufficient resolution.
For example, if minimum-linewidth features are situated in a peripheral portion of a mask subfield, the degree of blur of pattern segments can be at the b3 level in peripheral regions of the corresponding transfer subfield WSf, as shown in FIG. 4(b). As a result, the degree of blur is greater in those segments of the transfer subfield including minimum-linewidth features than in other segments of the transfer subfield.